Power amplifiers (PAs) are used in a wide variety of communications and other electronic applications. A major consideration in the design of power amplifiers is the efficiency thereof. It is generally desirable for linear power amplifiers to amplify radio frequency (RF) signals in a highly efficient manner. High efficiency is generally desirable so as to reduce battery drain in portable equipment, and the amount of power that is dissipated as heat. Linearity is generally desirable so that, for instance, the amplified signal contains no distortions and does not extend into adjacent frequency spectrum where it may cause interference with ongoing communications.
However, there are tradeoffs between maximum efficiency and high linearity. Specifically, efficiency is generally proportional to the input drive level, and high efficiency is usually not attained until an amplifier approaches its maximum output power, at which point the linearity is significantly degraded. Moreover, where the power amplifier is driven by a input signal having varying amplitude, a conventional class AB or B power amplifier, for example, must typically be operated at or near an average power level that is significantly below its saturation level to accommodate the peak input signal swings. As a result, the efficiency suffers.
The efficiency of conventional linear RF power amplifiers varies with the signal amplitude (envelope), resulting in relatively low average efficiencies, especially when the peak-to-average ratio is high. For example, for a Rayleigh-envelope (multicarrier) signal with a 10-dB peak-to-average ratio, the average efficiencies of ideal class A and B are only 5 percent and 28 percent, respectively. Various techniques for high-efficiency linear amplification (e.g. Kahn, Chiereix and Doherty) have been developed, but are subject to limitations in bandwidth or the dynamic range over which the efficiency is improved.
FIG. 1 is a circuit diagram schematically illustrating of a MOSFET class AB amplifier having a Gate (G), a Drain (D) and a Source (S). The amplifier is supplied with a supply voltage VDD and it receives an input signal Sin on the gate G, A current iDS will flow from the drain to the source depending on a gate-source voltage VGS and a drain-source voltage VDS. Said amplifier parameters, together with the input signal Sin(t), will control the current iD delivered to a resistive load RL that is connected to the drain. The output power Pout delivered from said load is depending on iD.
The vi-characteristics of the amplifier circuit is illustrated in FIG. 2a. The abscissa indicates the drain-source voltage and the ordinate the output current. In this example, peak-value for the drain source voltage is 2 VDD and the peak-value for the output current iD=VDD/RL, where VDD is the supply voltage and RL is the resistance of the load. Said peak-values define two values for the load line, having a slope −1/RL. The dashed graphs marked vGS=5 down to vGS=0 define the output current iD as a function of vDS and indicated values for vGS. As known for those skilled in the art, amplifier circuits may be operated in two different modes or regions, the active and the saturation region. Two regions are indicated by a dashed-dotted-line. As long the drive signal, i.e. the drive signal, the amplitude swing of said signal is located in the active region, the amplifier's response will be linear, i.e. the relation between the amplitude of the drive signal and the amplitude of the output signal is a linear function. However, in the saturation region said relation is not linear. As well-known, this non-linear function has a number of serious drawbacks.
FIG. 2b is a diagram illustrating an imaginary variation of the amplitude of a drive signal Sin(t), e.g. a Radio Frequency (RF) signal. For a RF signal, the peak-to-average swing may be as high as 7-10 dB. If the swing is high, exceeding over a certain amplitude level. Alimit, and the input voltage is backed-off, the amplifier starts operate in saturation and the output will become non-linear. Therefore, it would be desirable to increase the dynamic power range where the amplifier operates linearly for a full input amplitude swing with improved, maximum efficiency.